Alan Nathan on Humidors and Home Runs

Coors Field in mile-high Denver has been long viewed as a batter’s paradise and a pitcher’s nightmare. Because the air density in Denver is approximately 80 percent of that at sea level, fly balls hit there carry farther and pitches thrown there have less movement, both of which contribute to an increase in a variety of offensive statistics, particularly home runs. For the first seven seasons at Coors, there were 3.20 home runs hit per gamecompared to 1.93 per Rockies away game.

However, beginning in 2002 the Colorado Rockies began to store their baseballs in a humidor at a constant 50 percent relative humidity and 700F, as opposed to the more typical 30 percent humidity in Denver. During the period from 2002-2010 the Coors ratio decreased to 2.39, a reduction of 25 percent, while the away game ratio stayed constant at 1.86.

Is it plausible that the reduction in home runs can be attributed to the humidor? The primary goal of this article is to answer that question.